Code division multiple access (CDMA) has been extensively used in such applications as cellular and satellite communications. CDMA signals increase the spectrum required for the transmission of a particular data rate by modulating each information symbol with a spread spectrum code having a rate larger than the data rate. The same spreading code is used for each information symbol. Typically, the spreading code comprises of a few tens or a few hundreds elements, called chips. To decrease the correlations among spreading codes assigned to different users and thereby reduce the interference among different users, the data stream after spreading is typically scrambled with a pseudonoise (PN) code that is generated serially and cyclically and has a larger period than the spreading code. Examples of such CDMA signal spreading are the schemes used by the IS-95/CDMA2000 and 3GPP systems.
With CDMA, the signals from all users simultaneously occupy the same frequency band. The receiver discriminates the multiple signals by exploiting the properties of the spreading and scrambling codes that are applied to the signal of each user. The receiver attempts to match in time with the codes of the desired signal a replica of those spreading and scrambling codes. Only then the demodulation result is meaningful; otherwise it appears noise-like. Thus, if the arriving signals have different codes or different code off-sets, they can be discriminated at the receiver.
In the forward link of cellular communication systems, i.e. the communication from base stations to mobile terminals, the wireless channel may introduce multipath propagation. Even if the signals transmitted by the base station are spread using orthogonal codes (Walsh codes), the multipath propagation will destroy the orthogonality and produce multiple-access interference (MAI).
Interference cancellation (IC) attempts to suppress the MAI by estimating and subtracting the interference from the received signal, as disclosed in U.S. Pat. No. 5,553,062 to Schilling. Because the capacity of CDMA systems is typically MAI limited, estimating and canceling the MAI will increase the capacity. Alternatively, IC can reduce the symbol or frame error rate thereby allowing communication with higher data rates.
Equalization also attempts to suppress the MAI by restoring the orthogonality of the transmitted signals at the receiver. This is accomplished by inverting the effects introduced by the channel due to multipath propagation, as disclosed in “Multiple Access Interference Suppression with Linear Chip Equalizers in WCDMA Downlink Receivers”, K. Hooli, et. al, pp. 467-471, Globecom 1999.
Conventional IC schemes use knowledge of the spreading and scrambling codes used to transmit the signals, the decisions from a Rake receiver, and a channel estimate to reconstruct the components of the received signal and remove the interference, as disclosed in U.S. Pat. No. 5,553,062 to Schilling. The Rake receiver is however highly suboptimal in the presence of interference, particularly for higher order data modulations such as QAM. The data decisions provided by the Rake have poor reliability and when used by IC to reconstruct the interference they can result in significant performance degradation since the estimated received signal components are not very accurate. Subsequent cancellation of the estimated interference can therefore result in worse performance than the one of the Rake receiver since additional interference may be introduced due to erroneous decisions.
Conventional equalization methods try to either adapt to a known signal, such as the common pilot signal transmitted in the downlink of CDMA systems, or attempt to estimate or adapt to the channel impulse response and then use it to reverse the impact of multipath propagation on the received signal. Examples of well-known equalizers based on adaptation are the NLMS and its variants while equalizers based on channel inversion are the MMSE and its variants, as disclosed in “Multiple Access Interference Suppression with Linear Chip Equalizers in WCDMA Downlink Receivers”, K. Hooli, et. al, pp. 467-471, Globecom 1999. Although outperforming the Rake receiver in interference environments, equalization methods perform worse than IC when interference from signals transmitted to other mobile users in the same cell is low to moderate and generally have similar performance to IC in high interference environments.